1. Field of the Invention
This invention relates generally to a small electric motor, and more particularly to a small electric motor wherein adhesive reservoirs are provided at positions where commutator segments fitted to the small electric motor do not make sliding contact with brushes, that is, at the base portion of the commutator segments from which commutator terminals are led out for external connection, and an electrically conductive adhesive is injected into the reservoirs and deposited in such a manner as to bridge the adjoining commutator segments.
2. Description of the Prior Art
Sparks generated between commutator segments and brushes during rotation have heretofore been the cause of unwanted phenomena, such as damage to brushes or commutator segments due to electrical wear, and electrical noise. Consequently, a great number of means have so far been proposed to prevent spark generation. Among such means, a method of connecting capacitors, linear resistors, or non-linear resistors such as varistors across each commutator segment is generally considered effective.
These measures, however, involve an increase in the number of components, leading to increased manufacturing costs. Furthermore, variability in the characteristics of components, particularly those of varistors, poses a problem. To solve these problems, the present inventor has proposed a means of easily preventing spark and electrical noise generation by interposing electrically conductive grease between each commutator segment and setting the electrical resistance values of the electrically conductive grease within a proper range.
Where the motor is required to rotate at an exceptionally high speed, however, the electrically conductive grease, which is a viscous fluid, is very apt to splash away due to centrifugal force. This may cause not only contamination of the inside of the motor but also changes in the amount of deposition of the electrically conductive grease between each commutator segment, making it difficult to maintain electrical resistance between commutator segments within a proper range. At high temperatures, this tendency is further aggravated because the lowered viscosity of the grease increases the amount of splash.
Another spark quenching means using electrically conductive adhesive in place of the electrically conductive grease has also been considered. The electrically conductive adhesive, which hardens in a relatively short period after application, has the advantages of not splashing away due to centrifugal force and contributing to securing the commutator segments to the insulating cylinder. However, injecting the electrically conductive adhesive into narrow gaps between the commutator segments in such a manner that the adhesive does not cover the sliding surface of the commutator segments nor builds excessively up above the sliding surfaces of the commutator segments involves dexterity and careful attention. If the electrically conductive adhesive is accidentally applied to the sliding surfaces of the commutator segments or deposited excessively up above the sliding surfaces, the excess adhesive must be wiped off. This requires additional labor. Furthermore, a problem which is common between the electrically conductive adhesive and grease is that the sliding contact of the brush with the commutator segments causes the abrasion products of the electrically conductive adhesive or part of the electrically conductive grease to stick to the sliding surfaces of the commutator segments, deteriorating the continuity between the commutator segments and the brushes, or causing spark generation in some cases, quite contrary to original expectations. This can be prevented by injecting electrically conductive adhesive or grease into gaps between the commutator segments at a position where the commutator segments do not make contact with the brushes. In a small electric motor whose compact structural design requires dimensional limitations on each component, however, the length of the commutator segments in the axial direction of the motor is as short as a few millimeters. This makes it difficult to inject electrically conductive adhesive into the gaps between the commutator segments at a position where the commutator segments do not make contact with the brushes. In addition, limited space makes it impossible to apply the required amount of electrically conductive adhesive or grease.
Meanwhile, in order to set the resistance of the electrically conductive adhesive bridging the commutator segments to a predetermined value in accordance with the characteristics of the motor, the resistance of the electrically conductive adhesive can be changed by varying the amount of the electrically conductive substance to be added to the electrically conductive adhesive, or preferably, by varying the coating amount of a single type of electrically conductive adhesive. With the aforementioned structure of the motor, however, several types of electrically conductive adhesives having different resistance values have to be used because of limited coating space and accordingly the limited coating amount of electrically conductive adhesive.